The present invention relates to pressure and vacuum swing adsorption processes for the separation and recovery of certain gaseous components such as carbon dioxide from hot gas mixtures containing a substantial quantity of water vapor.
Many chemical, metallurgical and energy producing processes result in the release of carbon dioxide into the atmosphere which causes various environmental problems such as global warming. If the consumption of fossil fuels continues to grow at the present rate, it is estimated that the carbon dioxide discharged by the burning of fossil fuels will by itself raise the average atmospheric temperature of the earth by several degrees over the next thirty to sixty years. Thus, it is desirable to remove and/or recover carbon dioxide from these gases for environmental reasons. It is also desirable to separate and recover carbon dioxide from various mixed gases for further uses, e.g., the manufacture of liquid CO.sub.2, or use of CO.sub.2 as a chemical feedstock for the manufacture of other chemical products such as methanol.
Known ways of separating and recovering carbon dioxide from mixed gases include (1) selective absorption of carbon dioxide by a physical or chemical solvent; (2) selective permeation of carbon dioxide through a polymeric membrane; and, (3) selective adsorption of carbon dioxide by a pressure or vacuum swing adsorption process.
Separation of a mixed gas under the first method, selective absorption, can produce a carbon dioxide product having a high degree of purity, e.g., greater than 99% carbon dioxide, and a resulting gas mixture that is virtually free of carbon dioxide, e.g., less than 100 p.p.m. Generally, solvents utilized under selective absorption methods may be regenerated by heating with steam, which produces a carbon dioxide-enriched product stream. Typical systems for separating carbon dioxide from mixed gases utilizing selective absorption are described in "Gas and Liquid Sweetening," 2.sup.nd ed., pp. 98-155, by Dr. R. N. Maddox and published by John M. Campbell, 1974.
The second method, membrane separation, attains the separation of carbon dioxide from a mixed gas by setting in place in the flow path of the mixed gas a membrane capable of selectively permeating carbon dioxide, differentiating the pressure across the membrane, and passing the mixed gas through this membrane. Separation of a mixed gas under this method generally produces a carbon dioxide-enriched, but not pure, product at low pressure and a high-pressure effluent gas containing a dilute amount of carbon dioxide, e.g., less than 2 mole percent. A technique and apparatus for such membrane separation of gases is described in "Spiral-Wound Permeators For Purification and Recovery," pp. 37, by N. J. Schell and C. D. Houston, Chem. Eng. Prog., 33 (1982).
The third method, pressure or vacuum swing adsorption, effects the separation of carbon dioxide from a mixed gas by a procedure which comprises compressing the mixed gas and contacting it with an adsorbent, as for example, zeolites or activated carbons, thereby inducing selective adsorption of carbon dioxide, and then reducing the pressure, thereby desorbing the adsorbed carbon dioxide. Under pressure swing adsorption systems, CO.sub.2 is selectively picked up from the mixed gas at high CO.sub.2 partial pressure by the adsorbent, and the CO.sub.2 is released from the adsorbent by lowering the superincumbent gas phase partial pressure of CO.sub.2. A pressure or vacuum swing adsorption system can be designed to produce an essentially carbon dioxide-free stream, e.g., less than 100 p.p.m., at feed gas pressure and a carbon dioxide-enriched stream, e.g., 30-99 mole percent, at near ambient pressure. It should be understood by those skilled in the art that if the adsorption step is performed at superambient pressure and the desorption step is performed at or near ambient pressure, it is known as pressure swing adsorption. If the adsorption step is performed at or near ambient pressure and the desorption step is performed at subambient pressure, it is known as vacuum swing adsorption. For purposes of this application, the generic term pressure swing adsorption also includes the term vacuum swing adsorption. Two pressure swing adsorption processes for the separation of methane and carbon dioxide gas mixtures are described in "Separation of Methane and Carbon Dioxide Gas Mixtures by Pressure Swing Adsorption," pp. 519-528, by S. Sircar, Separation Science and Technology, Vol. 23 (1988).
The first and second methods are generally utilized when the feed gas mixture is available at a high pressure, e.g., greater than 300 p.s.i.g. An example of the application of either the first or the second method is the removal of bulk carbon dioxide, e.g., 10% to 30% carbon dioxide, from natural gas at a high pressure of approximately 700 p.s.i.g. The third method is generally utilized when the feed gas mixture is available at a low to moderate pressure, e.g., between about 10 to 250 p.s.i.g. An example of the third method is the removal of bulk carbon dioxide, e.g., 40% to 60% carbon dioxide, from a landfill gas at a pressure of approximately 100 p.s.i.g.
Each of the above mentioned methods is effective in separating bulk carbon dioxide from a gas mixture so long as the separation is conducted at or near ambient temperature, e.g., 15-40.degree. C. These methods are also most efficient when the feed gas contains less amount of water vapor. These methods are generally very ineffective in separating carbon dioxide from mixed gases at higher temperatures, e.g., in excess of 150.degree. C. For example, under the first method discussed above, the absorptive capacity of chemical solvents decreases significantly at high temperature. The method of polymeric membrane separation cannot be carried out at elevated temperatures, e.g., in excess of 200.degree. C., because the membrane will melt. Some microporous ceramic membranes (silica and alumina) are being developed for selective permeation of CO.sub.2 from a hot gas. Such membranes are described in "New Pore Size Control Of SiO.sub.2 Membrane," pp. 275-280, by Y. Ohshima, Y. Seki and H. Maruyama, Key Engrg. Materials, Vol. 159 (1999). These membranes, however, are at best enrichment devices in that they provide only low to moderate CO.sub.2 permiselectivity and thus would not be suited for recovery of pure CO.sub.2 or production of CO.sub.2 -free product.
Under pressure or vacuum swing adsorption, even in the absence of water vapor, the adsorption capacity and selectivity of conventional physical adsorbents such as alumina, silica gels, zeolites or activated carbon, decreases exponentially as the temperature of the feed gas mixture increases, making the separation process impractical. Even at or near ambient temperature, some of these conventional adsorbents such as alumina and silica gels, and zeolites become ineffective at CO.sub.2 removal where there is even a small amount of water vapor present in the feed gas. The water vapor must be removed from the feed gas prior to carbon dioxide separation.
There is a need for the removal of bulk carbon dioxide from effluent gases at high temperature where a high water vapor content may be present such as in chemical, metallurgical and power generation industries, as mentioned above. For example, in an oxygen-blown coal-fired power plant, the gasifier effluent after desulfurization, water gas shift reaction, and partial cooling to 300-400.degree. C. contains approximately 38.5% CO.sub.2, approximately 7.1% CO, approximately 52.3% H.sub.2, approximately 1.5% N.sub.2, approximately 0.1% H.sub.2 O and approximately 0.005% (H.sub.2 S+COS) at approximately 310 p.s.i.g. This heated gas is then expanded in a turbine for power generation. If the gas is cooled for CO.sub.2 removal by conventional methods, it must thereafter be reheated for expansion prior to being utilized in the turbine. This results in an enormous amount of heat exchange which requires a significant amount of apparatus and cost. Moreover, heat exchangers are somewhat inefficient and each time the mixed gas is cooled and reheated, there is some loss of heat, e.g., 20 to 30%, which cannot be recovered and utilized by the turbine. Thus, there is a need for CO.sub.2 removal and recovery from this gas at approximately 300-400.degree. C. without cooling. Under the prior art pressure or vacuum swing adsorption methods discussed above, CO.sub.2 removal and recovery cannot take place until the temperature of the effluent gases is first reduced to approximately ambient temperature.
As a second example, it is desirable to recycle into a furnace as feed gas at an elevated temperature without cooling, the effluent gas resulting from the metallurgical reduction of iron ore. The effluent gas resulting from the reduction process contains approximately 17% CO.sub.2, 27% CO, 50% H.sub.2, 2% H.sub.2 O, 3% CH.sub.4, 1% N.sub.2 and 100 p.p.m. H.sub.2 S and is at an elevated temperature of approximately 260.degree. C. and a pressure of about 45-60 p.s.i.g. In order to recycle this effluent gas to the reduction furnace as CO.sub.2 -free feed gas, it is necessary to remove CO.sub.2 without cooling. Under the prior art pressure or vacuum swing adsorption methods discussed above, CO.sub.2 removal and recovery cannot take place until the temperature of the effluent gases is first reduced to approximately ambient temperature. Thus, there is a large incentive for removal and recovery of CO.sub.2 from wet gases at the elevated feed gas temperature.
Other examples of methods for removal of CO.sub.2 from a hot gas by membrane separation are described in "Carbon Dioxide Removal With Inorganic Membranes," pp. 63-70, by R. R. Judkins and D. E. Fain, U.S./Japan Workshop on Global Change Research: Environmental Response Technologies, Honolulu (1993); U.S. Pat. No. 5,183,482; and European Patent Application Publication No. 0-723-802-A2.
It is desired to have an efficient process to remove carbon dioxide from hot gaseous mixtures containing water vapor without the necessity of cooling the mixtures.
It is further desired to have a process wherein the specific gaseous components can be converted with high efficiency into various useful organic substances, such as carbon dioxide, which can be reclaimed at a low cost.